There is known an explosion-proof electrical unit, such as a magnetic starter of the Wallacetown Co. of Great Britain (cf., "Electrical Equipment for the Mining Industry" pp. 3 and 4) in which elements of the electric circuit, such as contactors, automatic switches and transformers are accommodated in an explosion-protected equipment chamber closable by a cover plate to facilitate access to these elements for servicing and repairs. Apart from the equipment chamber, this construction of an explosion-proof electrical unit includes three separate explosion-protected chambers: a chamber for leading-in current conductors to lead-in a feeding cable, a chamber for leading out current conductors, for example, in the form of a plug-and-socket connector to lead out the cable connecting the explosion-proof electrical unit to a load, such as an electric motor, and a chamber for accommodating the isolator a drive mechanism of which is mechanically linked with the cover plate of the equipment chamber in such a manner that the cover plate is openable only with the disconnected isolator, and consequently in the absence of voltage at the elements of the electric circuit of the explosion-proof electrical unit. Cores of the feeding cable are electrically connected to the input terminals of the isolator via through current conducting terminals disposed at a blast-resistant wall between the explosion-protected chambers for leading-in the current conductors and isolator. The output terminals of the isolator are electrically connected to the elements of the electric circuit of the explosion-proof electrical unit via through current conducting terminals arranged at the blast-resistant wall between the explosion-protected chamber for leading-in current conductors and equipment chamber. The elements of the electric circuit of the explosion-proof electrical unit are electrically connected to the current conducting cores of the cable connecting this unit with the load via through current conducting terminals arranged at the blast-resistant wall between the explosion-protected equipment chamber and the chamber for leading out the current conductors. The blast-resistant chamber of the isolator is enclosed by a cover plate ensuring access from the outside to this chamber for assembling and servicing the isolator.
This prior art unit is complicated structurally, has a substantial weight and size, and is difficult to manufacture. These disadvantages are due mainly to excessive weight and size of the explosion-protected chamber of the isolator, provision of the partition wall with through current conducting terminals between the explosion-protected chambers of the isolator and equipment chamber, and cover plate of the explosion-protected chamber of the isolator with fastening members and explosion-proof flanges. In addition, excessive number of contact resistance at the points of connection of the current conductors, viz., at the through current conducting terminals from the chamber for leading-in current conductors to the chamber of isolator, at the input and output terminals and contacts of the isolator, and at the through terminals from the isolator chamber to the equipment chamber results in excessive heating of the elements inside the explosion-protected chambers and necessitates a reduction in the current loads or, while maintaining the rated current parameters of the unit, requires an increase in the rated current of the current conductors and elements of the unit to result in a larger overall size, heavier weight, and more labor consuming production of the explosion-proof electrical unit.
There is also known an explosion-proof electrical unit (cf., GB, A, No. 1,501,818) comprising an explosionprotected chamber for leading-in current conductors to lead-in a feeding cable, an explosion-protected equipment chamber accommodating elements of electric circuit of the unit, a cover plate of the explosion-protected equipment chamber, a blast-resistant wall dividing the explosion-protected chamber for leading-in current conductors and equipment chamber, through current conductiing terminals secured at the blast-resistant wall and intended to electrically connect the elements of the electric circuit of the unit with current carrying cores of the feeding cable, and a drive mechanism mechanically linked with the cover plate of the explosion-protected equipment chamber. The unit further includes an isolator, an explosion-protected chamber of the isolator arranged in the equipment chamber and having two cover plates on which there are mounted the through current conducting terminals for electrically connecting the output leads of the isolator with the elements of the electric circuit of the unit and with the current-carrying cores of the feeding cable.
However, this prior art construction of an explosion-proof electrical unit is bulky, heavy-weight and labor consuming in manufacture, since it includes an isolator, a separate explosion-protected chamber thereof, and a blast-resistant wall with the through terminals ensuring electrical connection between the output leads of the isolator and elements of the electrical circuit of the unit.
One characteristic feature of this explosion-proof unit construction is large number of connections between the current conductors causing substantial heating of the elements of the electric circuit of the unit and necessitating a larger size and weight of the unit. In addition, servicing of the isolator is difficult, because access thereto is possible only after opening the cover plate of the explosion-protected chamber for leading-in the current conductors, disconnecting the cores of the feeding cable and conductors in the equipment chamber from the through terminals at both cover plates of the explosion-protected isolator chamber, and removing the isolator from its chamber.
The invention aims at providing an explosion-proof electrical unit in which it would be possible to dispense with an isolator, as well as with an individual explosion-protected chamber thereof having through current conducting terminals for electrically connecting output leads of the isolator with elements of the electric circuit of the unit to thereby reduce the weight, size and amount of labor consumed for fabrication of the explosion-proof electrical unit.